The present invention relates generally to controllers for administering batch sterilization processes. In particular, it pertains to such a controller that provides on-line correction of a batch sterilization process when a temperature deviation occurs during the process.
Batch sterilization systems are widely used to sterilize shelf stable food products packaged in containers. In a typical batch sterilization system, a batch of these containers is placed inside the batch sterilizer of the system. Then, the controller of the system administers the batch sterilization process that is performed by the batch sterilizer on the batch of containers.
The batch sterilization process has come-up, processing, and cooling phases. These phases deliver a total lethality F to the batch of containers over a total time interval [t0, tc] covering these phases, where t0 is the begin time of the come-up phase and tc is the end time of the cooling phase. For purposes of this document, an open bracket [or] indicates that the corresponding time is included in the time interval while a closed bracket ( or ) indicates that the corresponding time is not included in the time interval. In order for the food product in the batch to be commercially sterilized, the total lethality actually delivered must satisfy a predefined target total lethality Ftargtot for the food product. The target total lethality may be set by the USDA (U.S. Department of Agriculture), the FDA (Food and Drug Administration), and/or a suitable food processing authority. Furthermore, some batch sterilization systems also include an optional requirement that the come-up and processing phases must deliver a heating lethality F to the batch over a heating time interval [t0, tp] that meets a predefined target heating lethality Ftargh for the food product, where tp is the end time of the processing phase. In this case, the operator sets the target heating lethality on an individual basis for each batch sterilization process.
As is well known, the lethality F delivered to the batch over a particular time interval [tm, tk] is given by the lethality equation:   Fo  =            ∫              t        m                    t        k              ⁢                  10                              (                                                            T                  CS                                ⁡                                  (                  t                  )                                            -                              T                REF                                      )                    /          z                    ⁢              ⅆ        t            
where tm and tk are respectively the begin and end times of the time interval [tm, tk], Tcs(t) is the product cold spot time-temperature profile of the product cold spot of the batch, z is the thermal characteristic of a particular microorganism to be destroyed in the sterilization process, and TREF is a reference temperature for destroying the organism. Thus, the heating lethality F delivered over the heating time interval [t0, tp]is given by this lethality equation, where tm=t0 and tk=tp. Similarly, the total lethality F delivered to the product cold spot over the total time interval [t0, tc] is also given by the lethality equation, but where tk=tc.
The time intervals [t0, tp] and [t0, tc] and the product cold spot time-time-temperature profile Tcs(t) must be such that the target lethalities Ftargh and Ftargtot are met by the heating and total lethalities F over [t0, tp] and F over [t0, tc]. In order to ensure that this occurs, various mathematical simulation models have been developed for simulating the product cold spot time-temperature profile Tcs(t) over the come-up, processing, and cooling phases. These models include those described in Ball, C. O. and Olson, F. C. W., Sterilization in Food Technology; Theory, Practice and Calculations, McGraw-Hill Book Company, Inc., 1957; Hayakawa, K., Experimental Formulas for Accurate Estimation of Transient Temperature of Food and Their Application to thermal Process Evaluation, Food Technology, vol. 24, no. 12, pp. 89 to 99, 1970; Thermobacteriology in Food Processing, Academic Press, New York, 1965; Teixeira, A. A., Innovative Heat Transfer Models: From Research Lab to On-Line Implementation in Food Processing Automation II, ASAE, p. 177-184, 1992; Lanoiselle, J. L., Candau, Y., and Debray E., Predicting Internal Temperatures of Canned Foods During Thermal Processing Using a Linear Recursive Model, J Food Sci., Vol. 60, No. 4, 1995; Teixeira, A. A., Dixon, J. R., Zahradnik, J. W., and Zinsmeister, G. E., Computer Optimization of Nutrient Retention in Thermal Processing of Conduction Heated Foods, Food Technology, 23:137-142, 1969; Kan-Ichi Hayakawa, Estimating Food Temperatures During Various Processing or Handling Treatments, J. of Food Science, 36:378-385, 1971; Manson, J. E., Zahradnik, J. W., and Stumbo, C. R., Evaluation of Lethality and Nutrient Retentions of Conduction-Heating Foods in Rectangular Containers, Food Technology, 24(11):109-113, 1970; Noronha, J., Hendrickx, M., Van Loeg, A., and Tobback, P., New Semi-empirical Approach to Handle Time-Variable Boundary Conditions During Sterilization of Non-Conductive Heating Foods, J. Food Eng., 24:249-268, 1995; and the NumeriCAL model developed by Dr. John Manson of CALWEST Technologies, licensed to FMC Corporation, and used in FMC Corporation""s LOG-TEC controller. A number of approaches have been developed for using these models to meet the target lethalities Ftargh and Ftargtot.
Referring to FIG. 1, a conventional approach is to use such a simulation model only for off-line (i.e., prior to administering the batch sterilization process) definition of a scheduled total time-temperature profile TsRT(t)0 for the batch sterilization process. In this approach, the controller of the batch sterilization system uses the simulation model to simulate a scheduled product cold spot time-temperature profile Tcs(t)0 that is predicted to occur over the come-up, processing, and cooling phases. This simulation is based on a pre-defined come-up time-temperature gradient TuRT(t), a scheduled processing retort temperature TpRT0, and a pre-defined cooling time-temperature gradient TcRT(t). The gradients TuRT(t) and TcRT(t) are based on heating and cooling temperature distribution tests conducted on the batch sterilizer and may include segments defined by endpoint temperatures and time durations.
The lethality equation described earlier is then used, where tm=t0 and tk=tp0, F=F0, and TCS(t)=TCS(t)0, to compute a heating lethality F0 that is predicted to be delivered over a scheduled heating time interval [t0, tp0] and is based on the scheduled product cold spot time-temperature profile TCS(t)0. Similarly, a total lethality F0 that is predicted to be delivered over a scheduled total time interval [t0, tc0] is computed based on the profile TCS(t)0 using the lethality equation, except where tk=tc0. As alluded to earlier, this is done so that the heating and total lethalities will meet the target lethalities Ftargh and Ftargtot.
By simulating the scheduled product cold spot time-temperature profile TCS(t)0 and computing the scheduled heating and total lethalities F0 over [t0, tp0] and F0 over [t0, tc0] in this way, the controller defines the scheduled total time-temperature profile TsRT(t)0 for which the target lethalities Ftargh and Ftargtot are satisfied. This profile TsRT(t)0 includes come-up, processing, and cooling portions over scheduled come-up, processing, and cooling time intervals [t0, tu0], (tu0, tp0], (tp0, tc0], respectively. The come-up and cooling portions comprise the portions of the gradients TuRT(t) and TcRT(t) over the corresponding scheduled come-up and cooling time intervals [t0, tu0] and (tp0, tc0], respectively. Similarly, the processing portion comprises the constant scheduled processing retort temperature TpRT0 over the scheduled processing time interval (tu0, tp0].
Moreover, some of the simulation models, such as the earlier mentioned NumeriCAL model and the models described in the Teixeira et al., 1969 and Manson et al., 1970 references use finite differencing. In this case, the scheduled product cold spot time-temperature profile TCS(t)0 and the predicted heating and total lethalities F0 over [t0, tp0] and F0 over [t0, tc0] are incrementally and iteratively simulated and computed.
The controller then administers the batch sterilization process to be performed by the batch sterilizer according to the scheduled total time-temperature profile TsRT(t)0. However, a temperature deviation may occur during the processing phase. This occurs when the actual retort temperature TaRT(tr) at each real sampling time tr during a deviation time interval [td, te) is below the scheduled processing temperature TpRT0. In this case, the heating and total lethalities F0 over [t0, tp0] and F0 over [t0, tc0] will in fact be less than the target lethalities Ftargh and Ftargtot.
In a conventional off-line scheduling approach, the controller has no means for on-line scheduling correction if a temperature deviation occurs. Thus, when such a deviation does occur, the operator is left with several undesirable options. The first option is to discard the batch entirely. However, this is wasteful and not necessary. The second option is to re-process the batch. This, however, will cause the food product in the batch to be over processed. And, the third option is to post process the recorded actual retort time-temperature profile TaRT(t) to determine whether the target lethalities Ftargh and Ftargtot have been satisfied. If they have not been satisfied, then the batch will be discarded or re-processed. If they have been satisfied, then the batch can be released for distribution. However, this is time consuming and, like the other options, wasteful and damaging to the food product.
In another approach, the controllers of the batch sterilization system is provided with conservative on-line scheduling correction capabilities. An example of such an approach is found in FMC Corporation""s LOG-TEC controller which uses the NumeriCAL model mentioned earlier. Referring to FIG. 2, this controller computes a scheduled total time-temperature profile TsRT(t)0 off-line using the model in the manner just described. And, while still off line, the controller also uses the model to generate a correction table of re-scheduled remaining time-temperature profiles TsRT(t)1, TsRT(t)2, etc. This table is then used for on-line correction of the scheduled total time-temperature profile TsRT(t)0 in case a temperature deviation does occur during the processing phase.
In generating the correction table, the controller selects a re-scheduled processing retort temperature TpRT1 that is below the scheduled processing retort temperature TpRT0. The controller then defines a corresponding re-scheduled remaining time-temperature profile TsRT(t)1 over a re-scheduled remaining time interval [tu1, tc1]. The re-scheduled remaining time interval comprises re-scheduled heating and cooling time intervals [t0, tp1] and [t0, tc1]. This is done in a similar manner to that just described. Thus, a product cold spot time-temperature profile TCS(t)1 is simulated that is based on the re-scheduled processing retort temperature TpRT1. From this product cold spot time-temperature profile, heating and cooling lethalities F1 over [t0, tp1] and F1 over [t0, tc1] are computed that satisfy the target heating and total lethalities Ftargh and Ftargtot and are predicted to be delivered over the re-scheduled heating and cooling time intervals. This entire process is then repeated for other re-scheduled processing retort temperature TpRT2, etc. to complete the correction table.
Then, if a temperature deviation occurs during the processing phase, the controller records the minimum actual retort temperature TaRT(tmin) at a particular sampling time tmin during the deviation time interval [td, te). The controller then locates the closest re-scheduled processing retort temperature TpRT1 in the correction table that is equal to or just below the retort temperature TaRT(tmin). The remainder of the processing phase is administered according to the re-scheduled remaining time-temperature profile TsRT(t)1 over the re-scheduled remaining time interval [tu1, tc1].
However, this approach can still cause the food product to be over processed. This is due to the use of the minimum actual retort temperature TaRT(tmin) during the temperature deviation for simulating the product cold spot time-temperature profile TCS(t)1. This simulation is overly conservative in that it disregards the fact that the actual retort time-temperature profile TaRT(t) was above the scheduled processing retort temperature TpRT over the time interval [t0, td) before the temperature deviation occurred. In other words, the portion of this product cold spot time-temperature profile over this time interval is overly conservative.
This means that, in the computation of the heating and total lethalities F1 over [t0, tp1] and F1 over [t0, tc1], full credit is not given to the lethality F that was actually delivered to the product cold spot of the batch over the time interval [t0, td) prior to the temperature deviation. As a result, the re-scheduled heating and total time intervals [t0, tp1] and [t0, tc1] are overly conservative since they are based on the overly conservative product cold spot time-temperature profile TCS(t)1. The food product in the batch will therefore be over processed since the heating and total lethalities actually delivered to the batch will substantially surpass the target lethalities Ftargh and Ftargtot, respectively.
Another disadvantage of this approach is that the minimum actual retort temperature TaRT(tmin) during the temperature deviation may be lower than any of the re-scheduled processing retort temperatures TpRT1, TpRT2, etc. in the correction table. In this case, the on-line scheduling correction just described will not be available. The operator of the batch sterilization system will then be only left with the options described earlier for the off-line scheduling approach.
In view of this, a new approach has been recently developed for on-line definition of the heating and total time intervals [t0, tp] and (tp, tc] using a finite difference simulation model. This approach is described in Teixeira, A. A. and Tucker, G. S., On-Line Retort Control in Thermal Sterilization of Canned Foods, Food Control, 8(3):13-20, 1997; Simpson, R., Almonacid S., and Torres, J.A., Computer Control of Batch Retort Process Operations, Food Processing Automation I, ASAE, 1991; Teixeira, A. A. and Manson, J. E., Computer Control of Batch Retort Operations with On-Line Correction of Process Deviations, Food Technology, p. 85-90, April 1982; and Datta, A. K., Teixeira, A. A., and Manson, J. E., Computer-based Retort Control Logic for On-Line Correction of Process Deviations, J. Food Sci., 51(2):480-483 and 507, 1986. This approach will also be discussed next to provide a better understanding of the differences between this approach and the approach used in the invention disclosed herein.
Referring to FIG. 3, in the on-line definition approach, the controller causes the batch sterilization process to begin without defining a scheduled total time-temperature profile TsRT(t)0 or a correction table. The come-up and processing phases are administered according to the pre-defined come-up time-temperature gradient TuRT(t) and the scheduled processing retort temperature TpRT. While these phases are being administered, the controller simulates for each current real sampling time tr the portion of a product cold spot time-temperature profile TCS(t) that has actually occurred over the time interval [t0, tr]. This is done based on the actual retort temperature TaRT(tr) measured at each real sampling time tr of the processing phase. From this portion of the cold spot time-temperature profile, the controller computes the heating lethality F actually delivered to the batch over the time interval [t0, tr] . This is done on-line at each real sampling time tr of the come-up and processing phases according to the lethality equation described earlier, where tm=t0 and tk=tr. The controller then determines whether this heating lethality satisfies the target heating lethality Ftargh. If it does not, then the process is repeated for the next real sampling time tr+xcex94tr, where xcex94tr is a pre-defined sampling period.
If the target heating lethality Ftargh is satisfied, then the controller uses the simulation model to simulate the portion of the product cold spot time-temperature profile TCS(t) that is predicted to occur over the cooling phase beginning at the current real sampling time tr. This is done while the controller is still on-line at the time tr. In doing so, the controller first defines a predicted cooling time-temperature profile TsRT(t) by shifting the cooling time-temperature gradient TcRT(t) so that it starts at the actual retort temperature TaRT(tr) at the time tr and occurs over a predicted cooling time interval [tr,tr+xcex94tc], where xcex94tc is the time duration of the predicted cooling time-temperature profile.
Moreover, while still on-line at the current real sampling time tr, the controller computes a total lethality F predicted to be delivered over a predicted total time interval [t0,tr+xcex94tc]. This is done by computing a cooling lethality F predicted to be delivered to the batch over the predicted cooling time interval [tr, tr+xcex94tc and adding it to the actually delivered heating lethality F over [t0, tr]. The predicted cooling lethality is computed according to the lethality equation, where tm=tr and tk=tr+xcex94tc, by using the portion of the product cold spot time-temperature profile TCS(t) predicted to occur over the time interval [tr, tr+xcex94tc]. If the predicted total lethality does not satisfy the target total lethality Ftargtot, then the controller repeats the entire process for the next real sampling time tr+xcex94tr.
If the target total lethality Ftargtot is satisfied, then the controller defines the time tr as the actual processing end time tp and the time tr+xcex94tc as the scheduled cooling end time tc. This means that the processing phase was administered over the actual processing time interval (tu, tp]. The controller then administers the cooling phase according to the now scheduled cooling time-temperature profile TsRT(t) over the correspondingly scheduled cooling time interval (tp, tc].
As discussed earlier, the on-line definition approach just described requires use of a finite difference simulation model. Such a model is required to accurately simulate the portion of the product cold spot time-temperature profile TCS(t) that actually occurs over each real time increment [trxe2x88x92xcex94tr, tr] of the processing phase using the actual retort temperature TaRT(tr) measured at the real sampling time tr. And, similar to the other approaches already described, this model can also be used to accurately simulate the portion of the product cold spot time-temperature profile TCS(t) predicted to occur over each simulation time increment [tsxe2x88x92xcex94tr, ts] of the cooling phase using the cooling retort temperature TcRT(ts) at the simulation sampling time ts.
A disadvantage to this on-line definition approach is that the definition of the processing and cooling end times tp and tc is open ended. In other words, the operator and the controller do not know the end times tp and tp in advance. This makes it difficult for an operator to comply with current FDA and/or USDA regulatory requirements in filing the batch sterilization process with the FDA and/or USDA.
Another disadvantage of this approach is that the product cold spot temperature profile TCS(t) must be simulated over each real time increment [trxe2x88x92xcex94tr, tr] and the heating lethality F over the time interval [t0, tr] must be computed at each real sampling time tr of the processing phase. This makes the approach computationally intensive and difficult to implement.
In summary, the present invention comprises a batch sterilization system, a controller for use in the batch sterilization system, and a method performed by the controller. The system, controller, and method are used to control and provide on-line correction of a batch sterilization process performed on a batch of containers. In addition to the controller, the batch sterilization system includes a batch sterilizer to perform the batch sterilization process on the batch of containers. The system also includes a sensor to sense actual retort temperatures in the batch sterilizer during the batch sterilization process.
The controller first defines a scheduled time-temperature profile for the batch sterilization process. The controller then compiles an actual retort time-temperature profile during the batch sterilization process from the actual retort temperatures sensed by the sensor. Before a temperature deviation has begun, the controller controls the batch sterilizer so as to administer an initial portion of the batch sterilization process before the temperature deviation has begun according to the scheduled time-temperature profile. The temperature deviation is between the actual retort time-temperature profile and a scheduled processing time-temperature profile.
In response to the temperature deviation, the controller defines a re-scheduled remaining time-temperature profile for a remaining portion of the batch sterilization process that begins when the temperature deviation clears. This is done by simulating the batch sterilization process based on the actual retort time-temperature profile. Furthermore, during the temperature deviation, the controller controls the batch sterilizer so as to administer corrections to clear the temperature deviation between the actual retort and re-scheduled remaining time-temperature profiles.
When the temperature deviation has finally cleared, the controller controls the batch sterilizer so as to administer the remaining portion of the batch sterilization process. This is done according to the re-scheduled remaining time-temperature profile.